Survivals of transplants between unrelated humans do not appear to correlate well with tissue antigen matching as defined by present testing methods. One proposed explanation for this discrepancy is the hot immune reaction against donor tissue antigens not identified by the present lymphocyte dependent assays. We are attempting to determine the influence of such antigens on graft rejection by a tissue to kidney bed grafting system. The state of rejection or acceptance in these grafts is determined by histological evaluation. We are studying the impact of the major mouse histocompatibility locus on rejection in the system as well as the influences of antigens which have not been previously defined as a major histocompatibility antigens. The donor or recipient origin of fibroblastic cells infiltrating tissue to kidney grafts in various states of rejection will be identified by flourescent labelling and chromosomal markers. Qualitative and quantitative relationships between repopulating cells and the time course of the rejection process will be studied. Both tissue slice rejection and host cell repopulation of the graft will be utilized to identify the genetic barrier(s) most important to the immune response of the host against a particular grafted tissue.